Ok,had a good read of the replies and I'll try my best to simplify my experiences.

Id say running your own business is like having two families and paying for both.

The rent,rates bills etc are like the other properties mortgage,the staff are like the children and basically you need to keep them all happy at the same time.

Added in are the bank manager,council,tax man and basically anybody else that feels you owe them something.

Things you take for granted,listen to the radio in work?that's £45 a year then please.couple of sets of overalls?£15-20 a month.want to put an extra 240v socket in?don't forget your commercial electrician then cos if someone finds out you did it yourself,you get fined.
Put the bins out?about £250 a year.
Go down the tip in your van?look forward to the council man fining you for your "duty of care".
Making a cup of tea?well,that's your electric then,teabags,sugar milk etc,take a shit,you pay the bogroll,not your boss.
£6-700 public liability insurance,£1500-2000 car insurance etc etc etc.

It all comes out of YOUR pocket.

THEN THERE'S THE PUBLIC!!!

Tell some/most of them it's £200 to change their timing belt and service their car and you'll swear you just told them you've bummed their 14yr old daughter,heaven forbid if they find the parts 1pence cheaper elsewhere,you a "rip off cunt" then.

Oh,and the ones who turn up asking you to take a "quick look" at something,like why their washers aren't working....because theres no fluid in them....and you put fluid in to test it.....and they drive off saying "ta",but give you fuck all,then tell their mates the same thing etc etc.

As I said,this was my experience,some people could never work for anybody else as they'd hate it,and some people do very well from it all.

My top tips for you from very hard learnt lessons.

1.no cash/no car/no exceptions."I'll drop it in tommorow",you'll get your fucking car back tommorrow then!!!see how quick they find it then.
2.people who pay up front,avoid unless it's a major job,you'll spend the cash on shite and get pissed off when they settle the bill and it seems shitty.
3.cash flow is king,it's a BIG reason my business died.
4.use autodata etc as you can prove to the customer then how long it SHOULD take.
If you do it quicker,good luck,slower,your fault.
5.be realistic with what you can do,to generate 8hrs solid production in a day means your working 10 at least,factor this in.
6.charge for absolutely everything,believe me it adds up so quick,after all,200ml of screenwash is still 200ml of screenwash,it's not free.
7.where there's muck,there's brass.
Dirty,shitty Mot welding style jobs bring in good coin,I was generating double what my staff were in a 1/3rd of the time,as people NEED mots,they must pay accordingly.chances are your about the 3rd or 4th place they've been if it's a shitty job and chances are the others have refused it.
We all want to do oil changes on a Cossie and then take it for a test drive with the owners blessing but his money is as good as the pizza delivery boy with a Nissan micras!!!
8.diversify/specialise.
Tough to call,but a specialist brings probably a better paying steadier flow of clients,yet is very boring whereas diversified work is interesting but sometimes you can get caught out through inexperience.
9.hammer out a good deal with the bank;no charges,fair overdraught etc.same goes for suppliers and so on.
10.avoid it being a "youth club".hangers on are a pain in the arse and cost you hugely in production.a 1/2hr chat is 1/2hr you need to maje up elsewhere,courteous and polite,but short and sweet.

Good luck if you do it,didn't work out for me but that's not to say it won't for others but just be brutally honest with yourself,work out all your costs to the penny,it's amazing how £2 here,£3 there soon makes £100 that's not been accounted for,and most businesses run such tight margins,it can be make or break.
One last piece of advice,take what people tell you who haven't been there themselves with a block of salt,they know shit all about how it feels,it's big pressure,or can be and you have to be a bit of a hard faced fucker to make it work,but there are rewards,just beware of the pitfalls.